The present invention is a method to determine bed levels in a reactor. In particular, the present invention uses a non-intrusive method to monitor the bed level in the reactor in a refinery. More particularly, the method is used to monitor the fluid lever in a delayed coker drum.
It is important to know the bed level in a process reactor, particularly is a delayed coker, in a refinery. Delayed-coking is an important process which is used to convert vacuum resids to lighter distillates and produce coke as byproduct. Delayed-cokers are operated in semi-batch mode, with at least two coke drums. While one is being filled or coked, the other is being de-coked. During the coking cycle, the drum is gradually filled with liquid feed at elevated temperature (typically around 900 degree F.) from a bottom nozzle, vapor product leaves the drum at the top, and coke forms inside the drum as the result of a complex solidification process. During the decoking cycle, the drum is first cooled by steam or water. After cooling and draining, a high-pressure rotary water jet is used to cut coke loose, which is removed from the bottom of drum for transportation. Delayed coker are recognized as one of primary capacity bottlenecks at many refineries.
Refineries that have difficulty in knowing the exact level in the drum run two risks. One is that they overfill the drum. In this case the coke inside the drum starts to stick to the top outlet and to any structure on the top of the drum. When this occurs the drum needs to be shut down and manually cleaned. This leads to significant throughput loss and maintenance cost. The second risk is that in order to prevent the condition described above a large portion of the drum is left unfilled and unused. This unused volume represents a loss of throughput for the unit. For example, a five foot excess outage in a 100 foot tall drum represents a 5% throughput loss.
Due to the significant financial impact of this problem a few different methods to predict or measure the level have been developed. The least expensive one is to calculate the level from the feed conditions. This suffers from the difficulty in knowing the final density of the coke and is not reliable. Methods using ranging radar or sonar have to be installed inside the reactor and suffer severe fouling problems. Other methods using radioactive sources have also been developed to measure the bed level. These are more expensive, have radioactive hazards, and generally suffer from reliability issues (probably due to workers avoiding maintenance contact with radioactive devices). Other drawbacks of the radioactive devices are that they can only provide discrete level measurements, instead of continuous measurements provided by the current invention.
The dynamic response of a vessel filled with a liquid/solid to the excitation of a sound source varies with the level and properties of material in the vessel. These variations in the dynamic response result from the changes in the natural frequencies, mode shapes and attenuations when the fill level varies. In the present invention, these differences are recognized and successfully correlated to different fill levels in a vessel. By measuring the dynamic response of the vessel to a known excitation source, the fill level could be estimated. The main advantage of this approach is that it is non-intrusive, low cost and simple to install and use.